80 Wild Birds and their Haunts 



be specifically lighter ; secondly, the circumference of their 

 thorax must be extended and the motions of their ribs 

 limited, that the muscles of the wings may have sufficient 

 space and firmness for their attachment. Both these 

 objects are attained by a modification of the apparatus ot 

 breathing. The lungs are highly vascular and spongy, 

 but they are not distended with air. 



" Tne weight of the body being a necessary con- 

 comitant of muscular strength, we see why birds, by 

 reason of their lightness, as well as by the conformation 

 of their skeleton, walk badly. And, on the other hand, in 

 observing how the lightness is adapted for flight, it is 

 remarkable how small an addition to their body will 

 prevent them rising on the wing. 



"It is interesting to notice the relations of great 

 functions in the animal economy. Birds are oviparous, 

 because they never could have risen on the wing had they 

 been viviparous. If the full stomach of a carnivorous 

 bird retard its flight, we perceive that it could not have 

 carried its young. The light body, the quill feathers, 

 the bill, and the laying of eggs are all necessarily con- 

 nected. 



' ' As everyone must have observed, the breastbone of 

 birds extends the whole length of the body, and, owing 

 to this extension, a lesser degree of motion suffices for 

 respiration ; so that a greater surface, necessary for the 

 lodgment and attachment of the muscles of the wings, is 

 obtained, whilst that surface is less disturbed by the 

 action of breathing, and is more steady. The vertebrae of 

 the back being fixed in birds, and the pelvis reaching 

 high, there is no motion in the body ; indeed, if there were 

 it would be interrupted by the sternum. We cannot but 

 admire, therefore, the composition of the neck and head, 

 and how the extension of the vertebrae and the length and 

 pliability of the neck, whilst they given to the bill the office 

 of a hand, become a substitution for the loss of motion in 

 the body, by balancing tne whole, as in standing, running 

 or flying. Is it not curious to observe how the whole 

 skeleton is adapted to this one object, the power of the 

 wings ? " 



